The invention relates to a method for navigating a terrestrial vehicle, in which a path length variable is measured using the number of revolutions of a wheel.
Such a method is used in a xe2x80x9cCarinxe2x80x9d navigation system, for example.
The demand for navigation aids has increased continuously in recent years. The aim here is to be able to indicate to the driver of a motor vehicle the best possible route for him to be able to reach the destination from a starting location. In this context, the overall task is divided into three parts, namely determining the position of the vehicle, route planning, and transmitting the route to the driver. The present invention relates to the first part, namely determining the position of the vehicle.
To determine the position of the vehicle, navigation satellites forming part of a global positioning system, or GPS, have been available for some time. Although the satellites can be used everywhere to determine a location or position with few restrictions, the accuracy which they are able to achieve is only within a range of 100 m. For inter-urban journeys, this may be sufficient in most cases. However, this xe2x80x9cinaccuracyxe2x80x9d is critical where the distance between roads is less than this magnitude, for example in towns or at traffic junctions, where a number of roads meet one another within a relatively short distance.
Hence, in particular fields, the use of xe2x80x9ccompound navigationxe2x80x9d has taken over, where the position of the vehicle is determined by ascertaining the direction and distance from a starting point. This type of navigation is also called xe2x80x9cdead reckoningxe2x80x9d.
For this distance measurement, it is possible to use a vehicle wheel, for example, which already runs on the ground. One revolution of the wheel then corresponds to a particular distance covered.
However, a prerequisite of this assumption is that the conditions of the wheel are and remain constant. This has been found not to be the case. There are a number of influencing variables in this context. It is easily imaginable that, in the case of air-filled rubber tires, for example, centrifugal forces act on the wheel when the vehicle is moving at relatively high speeds. Although the change in diameter and the associated change in the distance covered per wheel revolution is only in the percent range, when a distance of 10 km is covered, there is again an inaccuracy of approximately 100 m. A further influential factor is that the vehicle is raised or lowered to a greater or lesser extent by air currents, depending on its speed. This also has an effect on the tire circumference. Finally, thermal aspects must also not be overlooked. Heating of the tire, as may occur relatively quickly in summer, for example, and otherwise after a relatively long journey, can result in the tire diameter changing. The diameter also changes in the event of losses of tire pressure. Long-term changes result from wear on the tread.
The invention is based on the object of being able to use more accurate principles for navigation.
This object is achieved for a method of the type mentioned in the introduction by the path length variable being calibrated by means of at least one external position sensor.
It is thus no longer necessary to rely on the use of a fixed xe2x80x9cconversion factorxe2x80x9d between the number of revolutions of the wheel and the distance covered. Instead, this conversion factor can be changed if there is a need to do so. The distance covered can additionally be determined by using one or more external position sensors, for example satellites, and comparing the distance values obtained by means of this position sensor with the distance values obtained by means of the wheel. The information a obtained using the satellites is not used directly for position determination, but rather only indirectly to improve the accuracy of the measurement results.
Preferably, for calibration purposes, a first speed value is ascertained using the number of revolutions of the wheel and a second speed value is ascertained by means of the external position sensor, and the two speed values are used for calibration. As mentioned above, determination of the absolute position of the vehicle with respect to the external position sensor is likewise subject to certain inaccuracies. In addition, when satellites are used as position sensors, for example, a position signal can be obtained only at a frequency of approximately 1 Hz. However, the speed with respect to the position sensors can be obtained with a relatively higher degree of accuracy. If the speed is ascertained from the wheel revolution in the same way, two variables are readily obtained which can easily be related to one another.
In this context, it is particularly preferable for the second speed value to be determined by means of the Doppler effect. This provides a highly accurate procedure for determining the speed with respect to the position sensor. To be able to utilize the Doppler effect, it is sufficient to record a small number of oscillations of a signal output by the external position sensor, for example from a satellite. This provides information, at short intervals, about the current speed of the vehicle, which corresponds with a relatively high degree of accuracy to the actual speed of the vehicle.
Preferably, calibration is ongoing, at least in sections. This means that it is also possible to compensate for changes which arise during the journey, for example as a result of changing temperature conditions, speeds or the like.
Advantageously, one of the two speed values is classified in one of a plurality of classes, each of which comprises a predetermined speed range, and a calibration value in a class is ascertained using the speed values from this class. In this context, consideration is given to the fact that calibration on the basis of the number of revolutions of a wheel is more accurate the higher the speed, i.e., the more often the wheel turns per unit time. The reason for this is that the speed values determined by means of the Doppler effect are more accurate at higher speeds. If the xe2x80x9cconversion factorxe2x80x9d is now ascertained at a relatively high speed, then there is a high probability that this is more accurate than a conversion factor ascertained at a relatively low speed. This means that a speed dependency of the length per pulse can also be taken into account.
In this context, it is preferable if, at predetermined speeds, a calibration value from a predetermined higher speed class is used to ascertain the distance, if this calibration value is present. Hence, the higher accuracy at higher speeds is also utilized when the vehicle is traveling relatively slowly.
The case may arise, however, where calibration at a relatively high speed has not yet been possible, for example at the beginning of a journey. In this case, either a calibration from the lower speed class or from the lower speed range is used.
Advantageously, each calibration value is ascertained from a multiplicity of calibration operations. This increases the accuracy. If an error occurs in one calibration operation, this error is of only minor significance. By way of example, a calibration value may also be regarded as xe2x80x9cvalidxe2x80x9d only if a predetermined minimum number of calibration operations has taken place.
The calibration value is preferably formed by filtering the results of the individual calibration operations. This filtering smooths the curve for the results of the individual calibration operations and thus, makes the calibration value more uniform. This bears in mind that sudden changes in the distance covered per revolution are relatively rare. Hence, although corresponding measurement results may be taken into account should they arise, their influence on the calibration is small.
Preferably, calibration is not carried out if the acceleration ascertained by means of the external position sensor exceeds a predetermined acceleration value. In this case, the measurement results are discarded, for example. If the vehicle is accelerated or braked, the necessary information about the speed, for example, is not available with the required degree of constancy.
Preferably, the predetermined acceleration value is 1 m/s2. It has been found that a severe reduction in accuracy can be observed for acceleration values above this limit.
Preferably, calibration is not carried out if the speed ascertained by means of the external position sensor is lower than 5 m/s. This takes account of the fact that ascertaining the speed with respect to a number of external position sensors is subject to a systematic error which often has a fixed magnitude. The lower the speed, the greater the effect of this error. The higher the speed, the less significance can be attached to the error.
Preferably, the path length variable ascertained is a distance per pulse, the pulses being integrated until a new signal is available from the external position sensor. In this case, the fact that the signals from the external position sensor are output at predetermined intervals of time, for example at a frequency of 1 Hz, is used as a criterion for when calibration can be carried out again. This bears in mind that, when surrounding conditions are not favorable, for example, in the mountains or on a road with a lot of multistory houses, it is not always possible to receive the signal from the external position sensor. In this case, the intervals of time between individual calibration operations are simply lengthened appropriately until a signal is received from the external position sensor again or other, prescribed conditions have arisen.